In the art, there is still a need for polymer membranes having a well-defined selectivity and an improved resistance to harsh environments. Many separation processes would benefit from the option of being performed at higher temperatures than is currently feasible with state-of-the-art polymer membranes. In particular, anion exchange materials which are often based on polyethylenimine or on trimethylammonium derivatives of polystyrene have limited thermo-oxidative stability.
Highly thermo-oxidatively stable Brønsted basic membranes, after complexation with a strong acid, are also of great potential value in polymer membrane fuel cells. Such higher operating temperatures may lead to simpler fuel cell stack design and allow the use of less pure hydrogen as well as other fuels compared to Nafion®-based fuel cells. Nafion® perfluorosulfonic acid ionomers have the general structure (Butler, G. B.; O'Driscoll, K. F.; Wilkes, G. L. JMS-Rev. Macromol. Chem. Phys. 1994, C34(3), 325-373):

Another disadvantage of Nafion® membranes is that it requires 100% water saturation (i.e. a 100% RH environment) to achieve the required conductivity since water is the proton conducting phase. Hence, operation above 100° C. is virtually impossible due to the water loss from the Nafion® membranes, although an improved performance is expected at higher operating temperatures, e.g. a temperature in the range of about 120° C. up to about 150° C. Hybrid Nafion® membranes are also known in the art but have been applied without much success.
U.S. Pat. Nos. 5,525,436, 5,716,727 and 6,025,085 to R. F. Savinell and Morton H. Litt, incorporated by reference herein, disclose PBI (“PBI” means polybenzimidazole) and similar polymers doped with phosphoric acid. See also J-T Wang, J. S. Wainright, R. F. Savinell and M. H. Litt, J. Appl. Electrochem. 26, 751, 1996 and S. R. Samms, R. Wasmus and R. F. Savinell, J. Electrochem. Soc. 143, 1225, 1996. However, these systems have the disadvantage that they loose phosphoric acid during prolonged use which is presumably related to coagulation of the PBI by water from its phosphoric acid complex (cf. e.g. U.S. Pat. No. 7,045,241 to Akita Hiroshi and Komiya Teruaki, incorporated by reference herein, which discloses the isolation of a PBI composition by pouring its solution in polyphosphoric acid into water).
U.S. Pat. No. 6,723,757 to J. Kerres, A. Ullrich and T. Haring, incorporated by reference, discloses acid-base polymer blend membranes comprising as a first component either a cation exchanging polymer or an anion exchanging polymer and as a second component a polymer comprising one or more nitrogen containing basis moieties. However, pyridine as a nitrogen basic moiety is not disclosed.
U.S. Pat. No. 7,235,320 to G. Calundann, M. J. Sansone, O. Uensal and J. Kiefer, incorporated by reference, discloses a polymer membrane based on polyazoles wherein the azole moiety may be a pyridinylene moiety (structures XVI and XVII).
US 2006/0057449, also to G. Calundann, M. J. Sansone, O. Uensal and J. Kiefer, incorporated by reference, discloses a polymer membrane based on sulphonated polymers comprising recurring benzimidazole units.
US 2007/0141426 to S-w. Choi, H-y Sun, M-j Lee and W-s Jeon, incorporated by reference, discloses systems obtained by crosslinking polybenzimidazole with a benzoxazine-based monomer, wherein the nitrogen atom may be substituted with a 2-pyridyl or 3-pyridyl group.
There is, however, still a need in the art for improved polymer membranes.